The present invention relates to a surface mounting structure for a surface mounting electronic component, and more particularly to a surface mounting structure for a surface mounting electronic component wherein the surface mounting electronic component is joined on a land of a circuit board by soldering.
Since a solder joint has an excellent advantage in that a bonding operation is relatively easy, that bonding strength is reliably firm and that its cost is low, the solder joint has been used for various purposes in mounting an electronic component.
Meanwhile, in order to mount the electronic component in high density for miniaturization of an electronic device, the mainstream of a mounting structure for the electronic component is changing from an insertion mount that uses a lead component to a surface mounting that uses a surface mounting electronic component (chip component).
In a case where a surface mounting electronic component such as a ceramic condenser is mounted, as shown in FIG. 6A, a chip component 51 in the shape of a rectangular parallelepiped has formed on both ends thereof an electrode 52. In a state where a circuit board 53 has formed on the surface thereof a pair of lands 54, on each of which a cream solder 55 is printed, each electrode 52 of the chip component 51 is mounted on the cream solder 55 of the corresponding land 54. Subsequently, the cream solder 55 is melted (reflowed) in a reflow furnace by heat, thereby mounting the chip component 51 on the circuit board 53 by solder joints.
In a case where a service condition for the electronic device has temperature changes in a narrow range, normal solder joints are acceptable. However, a service condition for an electronic device for automobile use requires resistance to a temperature change between below zero and a few dozen degrees Celsius. That is, reliability in hot and cold environments and reliability in temperature cycling between the hot and cold environments are required.
In general, materials expand due to temperature rise and shrink due to temperature fall, and those degrees (coefficient of liner expansion) differ among the materials. Since the circuit board (land) side and the electronic component of a solder mounting portion for the electronic component have the different coefficients, thermal stress generates in a solder joining area between the circuit board side and the electronic component in accordance with temperature variations. As the generation of the thermal stress is repeated for an extended period, Lead(Pb)-rich α phase and tin(Sn)-rich β phase of solder composition spread thereby causing grain growth. The boundary between the crystal grains of the a phase and the β phase is fragile. Also, in a case where the land is made of copper (Cu), a Cu—Sn alloy layer is generated, however, this alloy layer is still hard and fragile. Cracks are generated mainly along such a fragile crystal grain boundary or an alloy layer. When the crack extends, the solder joining area may be broken thereby causing breakdown.
In order that breakage of the electronic component and deterioration of electrical characteristics are prevented, even if the thermal history such as heating or cooling increases during use of the electronic component mounted on the circuit board, a surface mounting structure for an electronic component is proposed such that the line width of the wiring pattern (land), the width of the electronic component, the distance between the lands, and the distance between electrodes of terminals meet a specific relation among them.
Japanese Unexamined Patent Publication No. 11-17308 discloses the surface mounting structure for an electronic component. As shown in FIG. 7, an electronic component 57 is joined on the pair of lands 54 formed on the circuit board 53 by solder 56. The surface mounting structure meets the relations 1.0≦Wh/Ws≦1.2 and 0.8≦Dh/De<1.0 where the line width of the land 54 is Wh, the width of the electronic component is Ws, the distance between the lands 54 is Dh, and the distance between the electrodes of the terminals is De. In the surface mounting structure for the electronic component, a heat shock test is conducted until 2000 cycles. The heat shock test defines a period where the surface mounting structure is alternatively immersed in solutions of minus 40 degrees Celsius and 130 degrees Celsius as one cycle. In the surface mounting structure for the electronic component which meets the relations, the number of electronic components which have been cracked on completion of 2000 cycles does not exceed 10% of the initial number (50) of electronic components, thereby being regarded as being sufficiently reliable.
The inventors of the present application have cut a specimen which was used in a temperature cycle test and have observed its cross section. As a result, as shown in FIG. 6B, a solder fillet 58 had a crack 59 first between the under surface of the electrode 52 and the land 54, and then the crack 59 extended slowly and finally extended through the solder fillet 58. It is noted that FIG. 6B shows one side of the chip component 51. The expression “the crack 59 extends through the solder fillet 58” means that the crack extends in a sheet so as to divide the solder joining area into a land side and an electrode side, thereby preventing electric continuity between the electrode and the land.
The chip component 51 and the land 54, which have different coefficients of liner expansion, tend to cause a stress concentration on the area between the under surface of each electrode 52 of the chip component 51 and the land 54 where the chip component 51 and the land 54 lie next to each other at the closest position. It is estimated that the stress concentration is particularly great at both corners on the middle side of the chip component 51 in the area between the under surface of each electrode 52 of the chip component 51 and the land 54.
The inventors of the present application have provided the solder fillet formed between the electrode of the electronic component and the land, not only in the position corresponding to the end face of the electrode, but also in the positions corresponding to both side faces of the electrode, thereby reducing the stress concentration on the. The idea of the inventors has been proved by performing the temperature cycle test of 3000 cycles. If the solder fillet is provided only in the positions corresponding to both side faces of the electrode, the land width only has to be increased compared to the width of electronic component. However, in a case where the land width is merely increased, when the solder joint is achieved by mounting the electronic component on the cream solder printed on the land and melting the solder by heat, the electronic component deviates from its desirable position.
While Japanese Unexamined Patent Publication No. 11-17308 discloses that the line width Wh of the land 54 is equal to or more than the width Ws of the electronic component and that the line width Wh of the land 54 is equal to or less than 120% of the width Ws of the electronic component, its object is directed to breakage of the electronic component 57 and prevention of deterioration of electrical characteristics without particularly referring to the prevention of the crack generation of the solder fillet. As to the deviation of the position of the electronic component, while the cited reference discloses that the thickness of the solder existing between the electrode of the electronic component 57 and the surface of the land 54 preferably ranges between 90 μm and 500 μm inclusive of 90 μm and 500 μm, the reference does not disclose any relation between the width of the land 54 and the deviation of the position.